dispatch_async in return method Objective-C

Question

I have been trying to use dispatch_async in a method that returns a result. However, I observed that the method returns before executing the dispatch_async block. Due to this I'm not getting the results I expect. Here is some code that demonstrates my problem.

-(BOOL) isContactExists {
    BOOL isContactExistsInXYZ = YES;
    UserId *userId = contact.userId;
    dispatch_async(dispatch_get_main_queue(), ^
    {
        iOSContact *contact = [iOSContact contactForUserId:userId];
        if (nil == contact)
        {
          isContactExistsInXYZ = NO;
        }
    });    

    return isContactExistsInXYZ;
}

This method isContactExists is called somewhere else and based on the response from that method I have to do some stuff. But every time, the value of isContactExistsInXYZ is not what I expect. How do I handle dispatch_async in this situation?

Answer 1

If your going the block route your method needs to look something like this.

- (void)isContactExistsWithCompletionHandler:(void(^)(BOOL exists)) completion 
{
    dispatch_async(dispatch_get_main_queue(), ^
    {
        BOOL isContactExistsInXYZ = YES;
        UserId *userId = contact.userId;
        iOSContact *contact = [iOSContact contactForUserId:userId];
        if (nil == contact)
        {
          isContactExistsInXYZ = NO;
        }
        completion(isContactExistsInXYZ);
    });
}

And where you are calling it something like this.

[someObject isContactExistsWithCompletionHandler:^(BOOL exists) {
    // do something with your BOOL
}];

You should also consider placing your heavy operations in a other que than main. Like this.

- (void)isContactExistsWithCompletionHandler:(void(^)(BOOL exists)) completion 
{
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, NULL);
    dispatch_async(queue, ^
    {
        BOOL isContactExistsInXYZ = YES;
        UserId *userId = contact.userId;
        iOSContact *contact = [iOSContact contactForUserId:userId];
        if (nil == contact)
        {
          isContactExistsInXYZ = NO;
        }
        dispatch_async(dispatch_get_main_queue(), ^
        {
            completion(isContactExistsInXYZ);
        });
    });
}